Document how to mark features as incomplete (#1151)
Document how to mark features as incomplete (#1151)

* Document how to mark features as incomplete

This was changed in https://github.com/rust-lang/rust/pull/86446 so that
incompleteness is included in the delcaration.

* Footerify link
rust-lang/rustc-dev-guide,rust-lang/rustc-dev-guide,rust-lang/rustc-dev-guide,rust-lang/rustc-dev-guide